1. Field of the Invention
The present invention relates to DC-to-DC power converter circuits, and more particularly, to a single-ended forward DC-to-DC converter having synchronous rectification and a circuit for resetting the transformer core.
2. Description of Related Art
Advancements in the electronic arts have resulted in increased integration of electronic devices onto reduced circuit form factors. This trend has driven a demand for power supplies that provide relatively low supply voltages, such as less than 3.3 volts. Such low voltage power supplies tend to have lower efficiency than higher voltage supplies due in part to the voltage drops across the semiconductor devices of the power supplies.
DC-to-DC converters are a type of low voltage power supply that converts an input DC voltage to a different output DC voltage. Such converters typically comprise a transformer that is electrically coupled via a switching circuit between a voltage source and a load. Converters known as single-ended forward converters include a single switch connected between the voltage source and the primary winding of the transformer to provide forward power transfer to the secondary winding of the transformer when the switch is on and conducting. A MOSFET device is typically used for the switch. It is also known in the art to utilize self-driven synchronous rectification in to provide relatively high efficiency of a DC-to-DC converter. Self-driven synchronous rectification refers to the use of MOSFET rectifying devices (i.e., rectifiers) having control terminals driven by the output voltages of the transformer secondary or auxiliary winding in order to provide the rectification of the output of the transformer.
A limitation of single-ended forward converters is that it is necessary to reset the transformer core to prevent saturation which means to discharge the magnetizing current of the transformer during the off period of the switch. This limitation results from the unipolar character of the transformer core excitation. More particularly, in the case of so-called "resonant reset" forward converters, when the switch turns off, energy stored in the magnetizing and leakage inductances of the transformer tends to resonate between the inductances of the transformer and the output capacitance of the switch, which generates voltage spikes and high-frequency ringing. Note that a MOSFET has an internal body capacitor (C.sub.M) between its drain and source terminals, and an internal body diode (D.sub.M) from its source to drain terminal. It is known to add an external capacitance across the switch to decrease the characteristic impedance of the device and the resonant frequency. This results in a reduction of the spikes and the ringing by significantly reducing the resonant frequency; however, the introduction of the additional capacitance disadvantageously increases the turn-on energy losses of the switch since the capacitor energies are dissipated when the main switch turns on. As a result, the efficiency of the converter is degraded.
Other known techniques exist for resetting the transformer of a single-ended forward converter while avoiding resonance with the switch capacitance. One such technique is to include an auxiliary winding of the transformer having inverted polarity and including a diode connected to the auxiliary winding in series. During the off period of the switch, the voltage across the switch goes to twice the voltage source as the diode becomes forward biased and conducts the magnetizing current back to the voltage source. This transformer resetting technique is referred to as "non-dissipative" since the magnetizing energy of the transformer is effectively recycled. Nevertheless, this technique also has an inherent limitation in that the maximum duty cycle of the converter is limited to 50% when a one-to-one primary to auxiliary turn ratio is used.
Another known transformer resetting technique is to include a resistor-capacitor-diode (RCD) network in parallel with the primary winding. The RCD network clamps the voltage on the switch to the minimal peak voltage consistent with a given source voltage and switch duty cycle, thereby eliminating the need for dead time while allowing for a wide range of duty cycles. This tends to reduce the voltage stress applied to the switch. Moreover, the transformer construction is simplified by avoiding the use of an auxiliary transformer winding. Nevertheless, this transformer resetting technique reduces the efficiency of the converter due to the dissipation of the magnetizing energy accumulated in the transformer during the on period of the switch. Instead of being recycled, this magnetizing energy is partially converted into heat by the RCD network.
An additional drawback of these non-dissipative and partly-dissipative transformer reset techniques is that there is a dead time while the primary switch remains open. During this dead time, the voltage across the switch equals the source voltage, so the voltage across the transformer equals zero and the magnetizing current either is equal to zero or is circulating in the opposite direction. The dead time increases undesirable voltage stress on the switch. Moreover, these transformer reset: techniques are also incompatible with the use of self-driven synchronous rectification, since the driving voltage of the free-wheeling rectifier is equal to zero during the dead time. This results in inefficiency of the converter, since inductor current is conducted through the body diode of the free-wheeling rectifier during the dead time.
Yet another method of transformer resetting is to use a series connection of a capacitor and an auxiliary switch connected across the transformer winding either on the primary or on the secondary side (referred to as an "active clamp"). When the main switch is turned off, the auxiliary switch is turned on, and vice versa. Thereby, magnetizing energy in the transformer is transferred to the clamping capacitor, and the clamping capacitor is resonating with the magnetizing inductance maintaining the necessary level of reset voltage. This active clamp reset provides non-dissipative reset of the transformer and minimal voltage stress on the main switch under steady state conditions as dead time is almost zero. For this reason, the active clamp method is compatible with self-driven synchronous rectification. Nevertheless, the driving voltage of the free-wheeling rectifier is highly variable (in reverse proportion to the line voltage), which may cause excessive gate losses of the free-wheeling rectifier at high switching frequencies, too low driving voltages in case of low output voltage at high line, or too high driving voltage at low line. Moreover, the active clamp method has other drawbacks, particularly under transient conditions (i.e., transitioning the line voltage from low to high or from high to low). Specifically, when the converter transitions from a low line voltage to a high line voltage, the main switch can be exposed to high voltage stress. Conversely, when the converter transitions from high line voltage to low line voltage, the transformer can be saturated as it takes time for the clamping capacitor to change its voltage.
Thus, it would be very desirable to provide a single-ended forward converter having a transformer resetting circuit that overcomes these and other drawbacks of the prior art, and which would be compatible with self-driven synchronous rectification. It would be further desirable to provide a single-ended forward converter able to accommodate transitions between high and low line voltage conditions.